Antibiotics can sometimes interact with other medicines or other substances. This means that the effects of one of the medicines can be altered by the other.

Some of the more common interactions are listed below. However, this is not a complete list.

If you want to check that your medicines are safe to take with your antibiotics, ask your GP or local pharmacist. You should also always carefully read the patient information leaflet that comes with your medicine.

Combined oral contraceptives

Antibiotics may cause your combined oral contraceptive pill to be less effective at preventing pregnancy.

Women taking combined oral contraceptives should use an extra method of contraception (for example, condoms) while taking the antibiotic and, in some cases, for seven days after finishing the course. Your GP will be able to advise you.

Medications

Some of the medications you may need to avoid or seek advice on if taking a specific class of antibiotic are outlined below.

Penicillin

It is usually recommended that you avoid taking penicillin at the same time as a medication called methotrexate, which is used to treat some types of cancers and severe autoimmune conditions such as the skin condition psoriasis. This is because combining the two medications can cause a range of unpleasant and sometimes serious side effects.

You may experience a skin rash if you take penicillin and a medication called allopurinol, which is used to treat gout.

Cephalosporins

Cephalosporins may not be suitable to take if you are also taking blood-thinning medications such as heparin and warfarin.

If you need treatment with cephalosporins, you may temporarily have to stop taking the blood-thinning medication.

Aminoglycosides

The risk of damage to your kidneys and hearing is increased if you are taking one or more of the following medications:

• antifungals - used to treat fungal infections
• cyclosporin - used to treat autoimmune conditions such as Crohn's disease and given to people who have had an organ transplant 
• diuretics - used to remove water from the body
• muscle relaxants

However, the risk of kidney and hearing damage has to be balanced against the benefits of using aminoglycosides to treat life-threatening conditions such as meningitis.

Tetracyclines

You should check with your GP or pharmacist before taking a tetracycline if you are currently taking any of the following medications:

• vitamin A supplements
• retinoids such as acitretin, isotretinoin and tretinoin used to treat severe acne
• blood-thinning medication
• diuretics
• kaolin-pectin and bismuth subsalicylate used to treat diarrhoea
• medicines to treat diabetes such as insulin
• atovaquone used to treat pneumonia
• antacids used to treat indigestion and heartburn
• sucralfate used to treat ulcers
• lithium used to treat bipolar disorder and severe depression
• digoxin to treat heart rhythm disorders
• methotrexate
• strontium ranelate used to treat osteoporosis
• colestipol or colestyramine used to treat high cholesterol
• ergotamine and methysergide used to treat migraines

Macrolides

It is highly recommended that you do not combine a macrolide with any of the following medications (unless directly instructed to by your GP), as the combination could cause heart problems:

• terfenadine, astemizole and mizolastine - which are all antihistamines used to treat allergic conditions such as hay fever
• amisulpride - used to treat episodes of psychosis
• tolterodine - used to treat urinary incontinence
• simvastatin - used to treat high cholesterol

Fluoroquinolones

You should check with your GP or pharmacist before taking a fluoroquinolone if you are currently taking any of the following medications:

• theophylline, which is used to treat asthma and also found in some cough and cold medicines
• the non-steroidal anti-inflammatory drug (NSAID) painkillers such as ibuprofen
• ciclosporin
• probenecid used to treat gout
• clozapine used to treat schizophrenia
• ropinirole used to treat Parkinson's disease
• tizanadine used to treat muscle spasms
• glibenclamide used to treat diabetes
• cisapride used to treat indigestion, heartburn, vomiting or nausea
• tricyclic antidepressants, such as amitriptyline, steroid medications (corticosteroids)

Some fluoroquinolones can intensify the effects of caffeine (a stimulant found in coffee, tea and cola), which could make you feel irritable, restless and cause problems falling asleep (insomnia).

Finally, you may need to avoid taking medication that contains high levels of minerals or iron as this can block the beneficial effects of fluoroquinolones. This includes:

• antacids
• zinc supplements
• some types of multivitamin supplements